Heat exchangers of the above kind are used in particular in motor vehicles having an internal combustion engine, so as to act either as a cooling radiator for the engine, or as a radiator or heat exchanger for heating the cabin of the vehicle. A cooling fluid, typically a mixture of water and glycol, flows through the tubes of the tube bundle in a closed circuit, being cooled by heat exchange with the air passing through the tube bundle outside the tubes.
It is conventional to provide the heat exchange tubes with a circular transverse cross section, both in their body and in their end portions. However, in order to optimise the performance of the exchangers of this type, it is also known to use heat exchange tubes in which the transverse cross section of the body is non-circular: this may simply be referred to as an oval cross section. This cross section is usually elliptical or similar, that is to say a planar cross section which is defined by two rectilinear parallel edges which are joined together through two semicircular edges.
In that case, the end portion of the tube usually has a transverse cross section which is of a different shape from that of the body of the tube. The end portion is generally less oblong than the body, and may typically be circular, so as to reduce the possibility of any deformation of the end portion. Since sealing between the end portion of the tube and the header plate is generally provided by means of a sealing gasket which is compressed between the end portion and a collar which surrounds the corresponding hole through the header plate, the pressure exerted by the gasket tends to cause flattening of the end portion, and thus deformation of the latter, if the end portion has a shape which is too markedly oblong.
Whatever the shape of the heat exchange tubes, and whatever type of header plate is used, it is necessary that the latter is as economical in material as possible, such that the pitch between any two adjacent holes for accommodating the heat exchange tubes is minimised, within either the same range of tubes or as between one range and the other. The material of the header plate lying between its holes must give sufficient mechanical strength to the header plate, and must be sufficient to enable a collar to be provided around each hole. Under these circumstances, it is difficult to optimise the tube pitches having regard to the dimensional constraints appropriate to the design of the heat exchanger, and in particular to that of its header plate.